1. Field of the Invention
The present invention relates to a process for preparing a carboxylic ester by carboxylating an olefin such as ethylene, propylene, butene, or the like in an aromatic hydrocarbon such as benzene, toluene, xylene, or the like in the presence of molecular oxygen and an aliphatic carboxylic acid, utilizing an improved catalyst.
2. Description of the Prior Art
It is known that noble metals of Group VIII of the Periodic Table, especially palladium, are effective as catalysts for the above-mentioned reaction (for example, Japanese Patent Publication No. 13081/1967). It also is known that bismuth and gold compounds, tin salts, organic phosphorous compounds and iron salts can be used as promoters for improving the catalytic activity of these catalysts (for example, U.S. Pat. No. 3,547,982 and Japanese Patent Preliminary Disclosure No. 18843/1972). Moreover, it has been customary to add a carboxylic acid salt of a metal of Group 1A or 2A of the Periodic Table to the catalyst as an additional activator. However, the above-mentioned catalyst combinations have not been sufficiently effective to permit industrial use of the reaction. Also, it has been attempted to increase the catalytic activity and lifetime of palladium since the latter is so expensive. In vapor phase reactions, the result of such attempts usually is a decrease in the catalytic activity after only a short period of operation.
It would be most desirable, therefore, to have a catalyst which has both high activity and a long lifetime.